1. Field of the Invention
The present invention relates to an optical fiber communication system using optical phase conjugation.
2. Description of the Related Art
By the application of nonlinear optics, it becomes possible to achieve new functions and improved characteristics unrealizable heretofore in the conventional optical technology. Particularly the use of optical phase conjugation enables compensation for phase distortion or chromatic dispersion in a transmission line. In Japanese Patent Application No. Hei 5(1993)-221856 (Optical Communication System with Compensation for Chromatic Dispersion, and Phase Conjugate Light Generator Applicable to Such System), there are already described a method which, by the application of the above characteristics to an optical fiber communication system, compensates for distortion of high speed optical pulses derived from group-velosity dispersion (GVD) in a fiber or nonlinear optical Kerr-effect, and also application of the method to wavelength-division multiplexed (WDM) optical transmission.
The conventional optical communication system is constructed by employment of optical components having linear optical characteristics, so that some limits are existent with respect to the characteristics and the functions though the construction is simple. In the field of the latest optical fiber communication system, there are practically realized a non-repeating system and a multistage optical amplifiers repeating system covering a long distance of several hundred to several thousand kilometers, and the transmission speed thereof is as high as several Gb/sec to 10 Gb/sec or more. Out of many existing problems to be solved in such systems, the most important and serious one is harmful influence of GVD in a fiber. In any of the systems mentioned above, the transmission characteristics are deteriorated by the influence derived from GVD and so forth to consequently induce restrictions on both the transmission speed and the transmission distance.
The principal countermeasure adopted in the prior art against GVD is based fundamentally on an idea to minimize the dispersion of the fiber itself. In the fiber realized as a result of the development, the dispersion is reduced to zero at transmission center wavelengths of 1.3 .mu.m and/or 1.55 .mu.m. Study of an optical modulator which generates signals less affected by GVD is also in progress, and a novel modulator using LiNbO.sub.3 is currently developed. Further advanced study is presently in progress with regard to a method of compensating for GVD in a transmission line by previously giving inverse chirping to transmission signal light or a method of carrying out compensation for dispersion either optically or electrically in a receiver. Thus, regarding the countermeasures against GVD, continuous researches and studies are advanced with respect to the entirety of transmitters, transmission lines and receivers due to reflection of such serious problems.
When signal light is composed of an optical pulse (inclusive of a pulse train consisting of a plurality of optical pulses) processed through intensity modulation or amplitude modulation, there may occur a phenomenon that the pulse waveform is distorted by some other reasons than GVD. Notable ones are assumed to be as follows:
(1) Waveform distortion caused by GVD and optical Kerr effect
(2) Waveform distortion caused by random phase fluctuation derived from accumulation of ASE (amplified spontaneous emission) noise of optical amplifier in a multistage optical amplifiers repeating transmission
Of the above two items, it is intended in the present invention to deal particularly with the waveform distortion in the item (1).
Suppose now that an optical pulse propagates in a dispersion medium. When an unchirped pulse passes through a normal dispersion medium (.differential..sup.2 .beta./.differential..omega..sup.2 &gt;0), the pulse is shifted toward a lower frequency side at its leading edge or is shifted toward a higher frequency side at its trailing edge. Meanwhile in the case of an anomalous dispersion medium (.differential..sup.2 .beta./.differential..omega..sup.2 &lt;0), the pulse is shifted toward a higher frequency side at its leading edge or is shifted toward a lower frequency side at its trailing edge. In the above, .beta. and .omega. denote the propagation constant and the angular frequency of the light, respectively. In a normal dispersion medium, the longer the wavelength, the faster the group velocity; whereas in an anomalous dispersion medium, the shorter the wavelength, the faster the group velocity. In either case, therefore, the pulse width is increased.
When the light intensity is great, the refractive index is changed by the following value due to the optical Kerr effect. EQU .DELTA.n(t)=n.sub.2 .vertline.E(t).vertline..sup.2 (1)
In the above equation, n.sub.2 is the amount termed nonlinear refractive index, and its value in the case of a silica fiber is approximately 3.2.times.10.sup.-20 m.sup.2 /W. When an optical pulse is affected by the optical Kerr effect in a nonlinear medium, the spectrum is chirped as following. ##EQU1##
In the above equation, .DELTA.z denotes the interaction length. This phenomenon is generally termed self-phase modulation (SPM). Due to this SPM, the optical pulse is shifted to a lower frequency side at its leading edge or is shifted toward a higher frequency side at its trailing edge. Because of the chirping caused by such SPM, the influence of the dispersion is rendered more noticeable to consequently increase the pulse distortion. Therefore, where the optical pulse is affected by the optical Kerr effect in a normal dispersion medium, the pulse is more broadened than in the case of dispersion alone; whereas in an anomalous dispersion medium, there occurs pulse compression. Accordingly, in considering the aforementioned effect of GVD as well, great pulse broadening occurs in a normal dispersion medium, whereas there appears, in an anomalous dispersion medium, either greater one of the pulse broadening derived from GVD and the pulse compression derived from SPM. An optical solution is obtained by balancing such two effects.
It is generally prone to be believed that, in an anomalous dispersion medium, a high signal-to-noise ratio can be advantageously retained by applying pulse compression derived from SPM. However, owing to the latest technique that enables satisfactory transmission with a high-level optical power by the use of a light amplifier and further realizes relatively small value of GVD due to development of dispersion-shifted fiber, it is not exactly certain now that application of pulse compression brings about a better result. In other words, a large waveform distortion is generated as the pulse compression effect is rendered excessive. Particularly in the case of NRZ pulses, concentrative pulse compression occurs at leading and trailing edges of the pulses, so that sharp waveform changes are induced and, in an extreme case, a fall portion passes a rise portion to eventually cause a phenomenon that one pulse is split into three. Meanwhile in long-distance light-amplified multi-repeating transmission, there exists a problem that four-wave mixing (FWM) is caused among the signal light, which acts as pump light, and spontaneous emission light from the light amplifier, hence exerting tremendous influence.